This study will investigate immunologic mechanisms responsible for maintaining the fetal maternal relationship during pregancy. Classical laws of transplantation would predict that the mother would reject the fetus. We propose to further elucidate the suppressive mechanisms which prevent fetal rejection. First, circulating complexes of antigens and antibodies (immune complexes) will be sought in blood of pregnant humans, the components isolated and purified and their potential role in blocking lymphocyte function tested. Gestational hormones, particularly progesterone, probably suppress maternal cellular immunity toward the fetus. The mechanism of this endocrinologic control of immunity during pregnancy will be investigated. Finally, cellular alloimmunity during pregnancy in the mouse and its potential suppression by a class of cells called suppressor cells will be studied. Our approach is multifaceted and employs technics of immunochemistry, endocrinology, cellular immunity and cell biology in order to better understand "nature's transplant."